The International Information Center for Geotechnical Engineers

Prefabricated Vertical Drains

 

Introduction

 

Prefabricated Vertical Drains (PVDs) or ‘Wick Drains' are composed of a plastic core encased by a geotextile for the purpose of expediting consolidation of slow draining soils. They are typically coupled with surcharging to expedite preconstruction soil consolidation. Surcharging means to pre-load soft soils by applying a temporary load to the ground that exerts stress of usually equivalent or greater magnitude than the anticipated design stresses. The surcharge will increase pore water pressures initially, but with time the water will drain away and the soil voids will compress. These prefabricated wick drains are used to shorten pore water travel distance, reducing the preloading time. The intent is to accelerate primary settlement. Pore water will flow laterally to the nearest drain, as opposed to vertical flow to an underlying or overlying drainage layer. The drain flow is a result from the pressures generated in the pore water. Figure 2 below demonstrate vertical water flow without the use of prefabricated wick drains, and horizontal water flow with the use of wick drains.

http://www.geosinindo.co.id/wp-content/uploads/2011/06/6-1.png[B] Cross-Section With and Without Vertical Drains

Initial settlements are seldom of much practical concern, except for loads on thick plastic or organic soils having marginal stability wherein large shear deformation may continue due to undrained creep [6].

Prefabricated wick drains have several other purposes. They may be applied to reduce potential down drag on piles, or increase storage capacity for future landfills and waste containment sites. A developing application for PVD are the collection and extraction of contaminated groundwater, which may be coupled with cutoff walls to ensure full withdrawal.

History

In the 1920s, a technique for installing sand drains, a PVD predecessor, was patented in the U.S. The California Division of Highways, Materials and Research Department conducted laboratory and field tests on vertical sand drain performance beginning in 1933. Within the decade Walter Kjellman, then Director of the Swedish Geotechnical Institute, developed a prefabricated band-shaped vertical drain made of cardboard core and paper filter jacket which was installed into the ground with mechanical equipment [9]. Cardboard wick drains, and subsequently paper-wrapped plastic drains, were installed outside of the U.S. though the 1970s. A decade after that, entirely plastic PVDs were introduced as a more durable, reliable, and inexpensive option over the sand drains. Because these plastic drains could be installed very quickly as compared to sand drains, by the late 1980s, they largely replaced sand drains [17].

 

 

A case study for the acceleration of primary consolidation using wick drains is used to show some numbers on the time required for consolidation with and without the wick drains.

 

Construction of the New Istana (Royal Palace) for the Sultan of Brunei was to be completed before Brunei became independent in July 1983. This construction required fill to be placed on very soft compressible floodplain soils. Computations made before the fill placement in 1981 predicted several feet of fill settlement would occur from the consolidation beneath the floodplain area. The predicted primary consolidation settlement was 8.3 feet. For the 60 foot thickness of compressible soil, 50% of the settlement was estimated to take 3 years to complete, and 90% of the settlement was estimated to take 13 years. These predicted consolidations were computed by the method outlined by Leonards (1962). This required time frame was unacceptable and needed to be accelerated so that 90% of the consolidation settlement is completed within 6 months. A viable course of action was to install wick drains on the site.

 

A center to center spacing for the wick drains of 5 feet was computed by the method outlined in Hansbo (1979), based on the requirement that 90% consolidation of the soft sediments occur within the above mentioned 6 months. One foot of fill was placed every 2 days so that there were 85 1-foot increments in 6 months. The use of wick drains indicated that primary consolidation settlement would be accelerated by a factor of about 25, and the secondary compression to not be affected. There were 12 feet of fill placed before installation of the wick drains, and a subsequent 1.5 foot thick drainage blanket placed on top of the fill. Fill placement started in mid-September 1981 and was completed by the end of March 1982.

 

Surface settlement markers and deep settlement gauges were installed throughout the floodplain to provide settlement data before, during, and after the fill embankment construction, enabling ongoing evaluations of the wick drain performance. Early readings observed an immediate response to the installation of the wick drains. The last reading was taken in July of 1982 and the maximum settlement occurred was 7 feet. It was assumed this represented 90% of the primary consolidation and that total primary consolidation settlement would be 7.8 feet. This was in good agreement with the predicted maximum primary consolidation of 8.3 feet.

 

The installation of wick drains in the soft floodplain soils allowed construction of the fill embankment to proceed on schedule and brought about the desired results, increasing rate of consolidation by a factor of 25.

 

 

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